1. Field of the Invention
The present invention is directed to the reduction of carbon dioxide emissions from Fischer-Tropsch GTL facilities.
2. Description of the Related Art
The conversion of remote natural gas assets into transportation fuels has become more desirable because of the need to exploit existing natural gas assets as a way to satisfy the increasing need for transportation fuels. Generally, the term xe2x80x9cremote natural gasxe2x80x9d refers to a natural gas asset that cannot be economically shipped to a commercial market by pipeline.
Conventionally, two approaches exist for converting remote natural gases into conventional transportation fuels and lubricants, including but not limited to, gasoline, diesel fuel, jet fuel, lube base stocks, and the like. The first approach comprises converting natural gas into synthesis gas by partial oxidation, followed by a Fischer-Tropsch process, and further refining resulting Fischer-Tropsch products. The second approach comprises converting natural gas into synthesis gas by partial oxidation, followed by methanol synthesis wherein the synthesized methanol is subsequently converted into highly aromatic gasoline by a Methanol To Gasoline (MTG) process. Both of these approaches use synthesis gas as an intermediate. Also, while other approaches exist for using natural gas in remote locations, such approaches do not produce conventional transportation fuels and lubricants, but instead produce other petroleum products including, but not limited to, liquified natural gas (LNG) and converted methanol.
The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into saleable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having high crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels.
Accordingly, in view of the above disadvantages of the Fischer-Tropsch and MTG processes, there is a need for a process that is capable of producing desirable Fischer-Tropsch petroleum products while significantly minimizing CO2 emissions commonly generated during the production of such products.
Catalysts and conditions for performing Fischer-Tropsch reactions are well known to those of ordinary skill in the art, and are described, for example, in EP 0 921 184A1, the contents of which are hereby incorporated by reference in their entirety. A schematic of a conventional Fischer-Tropsch process is shown in FIG. 1. A feed stream 11 comprising CH4, O2 and H2O is introduced into a synthesis gas formation reactor 13. Although feed stream 11 is depicted as a single stream, it may be desirable to introduce the feed as multiple separate streams. In fact, because it is undesirable to mix O2 and CH4 before introduction to the synthesis gas formation reactor 13, it may be especially beneficial to introduce at least the O2 and CH4 in separate streams. A synthesis gas stream 14 comprising CO, H2 and CO2 is produced from the synthesis gas formation reactor 13 and introduced into a Fischer-Tropsch reactor 15. A Fischer-Tropsch process is conducted to produce a Fischer-Tropsch product stream 16 that is fed into a first separator 17. The first separator 17 separates the Fischer-Tropsch product stream into an unreacted gas stream 18, comprising CO, H2 and CO2, and a hydrocarbon products stream 22 comprising principally C5+ liquids with small amounts of dissolved C1-C5 gaseous products. The unreacted gas stream 18 can be recirculated in a stream 21 to be mixed with the synthesis gas 14 before entering the Fischer-Tropsch reactor 15. In addition, a portion of the unreacted gas stream 18 can be removed in an exit stream 19 where excess CO, H2 and CO2 are ignited by a flare or used as low-BTU fuel.
The generation of CO2 emissions from Fischer-Tropsch processes can be understood by examining the stoichiometry of the reaction that occurs during a Fischer-Tropsch process. For example, during Fischer-Tropsch processing, synthesis gas (i.e., a mixture including carbon monoxide and hydrogen), is generated, typically from at least one of three basic reactions. Typical Fischer-Tropsch reaction products include paraffins and olefins, generally represented by the formula nCH2. While this formula accurately defines mono-olefin products, it only approximately defines C5+ paraffin products. The value of n (i.e., the average carbon number of the product) is determined by reaction conditions including, but not limited to, temperature, pressure, space rate, catalyst type and synthesis gas composition. The desired net synthesis gas stoichiometry for a Fischer-Tropsch reaction is independent of the average carbon number (n) of the product and is about 2.0, as determined by the following reaction equation:
nCO+2nH2 nH2O+nCH2
where nCH2 represents typical Fischer-Tropsch reaction products such as, for example, olefins and paraffins.
The three general reactions that produce synthesis gas from methane are as follows:
steam reforming of methane: CH4+H2O CO+3H2; dry reforming, or reaction
between CO2 and methane: CH4+CO2 2CO+2 H2; and partial oxidation using oxygen:
CH4+xc2xdO2 CO+2H2.
Although the above general reactions are the basic reactions used to produce synthesis gas, the ratio of hydrogen to carbon monoxide produced by the above reactions is not always adequate for the desired Fischer-Tropsch conversion ratio of 2.0. (In the instant application, all ratios are molar ratios, unless otherwise noted.) For example, in the steam reforming reaction, the resulting ratio of hydrogen to carbon monoxide is 3.0, which is higher than the desired hydrogen to carbon monoxide ratio of 2.0 for a Fischer-Tropsch conversion. Similarly, in the dry reforming reaction, the resulting hydrogen to carbon monoxide ratio is 1.0, which is lower than the desired hydrogen to carbon monoxide ratio of 2.0 for a Fischer-Tropsch conversion. In addition to exhibiting a hydrogen to carbon monoxide ratio that is lower than the desired ratio for a Fischer-Tropsch conversion, the above dry reforming reaction also suffers from problems associated with rapid carbon deposition. Finally, because the above partial oxidation reaction provides a hydrogen to carbon monoxide ratio of 2.0, the partial oxidation reaction is the preferred reaction for Fischer-Tropsch conversions.
Generally, the proportion of carbon in methane that is converted to heavier hydrocarbon products in Fischer-Tropsch/GTL processes is estimated to be about 68%. Thus, the remaining 32% of the carbon is left to form significant amounts of CO2. Although these estimated values were provided for a GTL facility using cryogenic air separation, an autothermal reformer, a slurry bed Fischer-Tropsch unit and a hydrocracker for converting heavy wax into saleable products, GTL facilities using alternative technologies would exhibit similar carbon conversion efficiencies and CO2 emissions. A detailed description of the above estimates is described in xe2x80x9cCO2 Abatement in GTL Plant: Fischer-Tropsch Synthesis,xe2x80x9d Report #PH3/15, November 2000, published by IEA Greenhouse Gas RandD Programme, which is incorporated herein by reference in its entirety,
In commercial practice, an amount of steam added to a partial oxidation reformer can control carbon formation. Likewise, certain amounts of CO2 can be tolerated in the feed. Thus, even though partial oxidation is the preferred reaction for Fischer-Tropsch conversions, all of the above reactions can occur, to some extent, in an oxidation reformer. It is also important to provide a low sulfur gas feedstock for the partial oxidation reformer. Typically this can be done by use of an adsorption or absorption process or combination thereof. Suitable adsorbents include, for example, water, amines, caustic compounds, combinations thereof and the like. Suitable adsorbents include, for example, ZnO, Cu, Ni, combinations thereof and the like. ZnO is a preferred adsorbent because it selectively removes sulfur species without substantially removing CO2.
During partial oxidation, carbon dioxide forms because the reaction is not perfectly selective. That is, some amount of the methane in the reaction will react with oxygen to form CO2 by complete combustion. The reaction of methane with oxygen to form CO2 is generally represented by the following reactions:
CH4+O2 CO2+2H2
and
CH4+2O2 CO2+2H2O.
Furthermore, steam added to the reformer to control coking, or steam produced during the Fischer-Tropsch reaction can react with CO to form CO2 in a water gas shift reaction represented by the following general reaction:
CO+H2O CO2+H2.
Thus, invariably a significant amount of CO2 is formed during the conversion of methane into transportation fuels and lubricants by the Fischer-Tropsch process. The CO2 produced during the Fischer-Tropsch process exits the Fischer-Tropsch/GTL process in a tail gas exiting the Fischer-Tropsch unit. Tail gases exiting a Fischer-Tropsch/GTL process comprise any gases that remain unconsumed by the Fischer-Tropsch process.
The above equations represent general stoichiometric equations; they do not reflect an optimum synthesis gas composition for the kinetics or selectivity of a Fischer-Tropsch reaction. Moreover, depending on the nature of the Fischer-Tropsch catalyst, synthesis gas ratios other than 2.0, typically less than 2.0, are used to prepare the feed to a Fischer-Tropsch unit. However, because Fischer-Tropsch units typically produce products exhibiting a hydrogen to carbon monoxide ratio of about 2.0, the limiting reagent, typically H2, is consumed first. The extra reagent, typically CO, is then recycled back to the Fischer-Tropsch unit for further conversion. Synthesis gas compositions having hydrogen to carbon monoxide ratios other than 2.0 are typically generated by recycling unused reagents.
In view of the above discussion, there is an urgent need for a process that can produce desirable Fischer-Tropsch/GTL process products while minimizing the CO2 emissions generally associated with Fischer-Tropsch/GTL processing.
The present invention satisfies the above objectives by providing a process that not only reduces CO2 emissions, but also produces desired Fischer-Tropsch/GTL petroleum products.
The process of the present invention reduces CO2 emissions by converting at least a portion of the CO2 emitted by a Fischer-Tropsch process into additional CO that can be converted into hydrocarbons. More specifically, the process of the present invention reduces CO2 emissions by reacting hydrogen by-product, generated from Fischer-Tropsch naphtha reforming, with CO2 in a feed stream in a reverse water gas shift reaction to convert the CO2 into additional CO that can be converted into hydrocarbons. Thus, one important advantage of the present invention is that it can produce desirable Fischer-Tropsch/GTL petroleum products and can economically reduce CO2 emissions produced during the production of such products without having to employ costly CO2 reduction measures.
In particular, a process, according to the present invention, for reducing CO2 emissions from a Fischer-Tropsch facility includes introducing a synthesis gas into a Fischer-Tropsch reactor and performing a Fischer-Tropsch process on the synthesis gas to obtain a Fischer-Tropsch product and CO2. At least a portion of the CO2 is fed from the Fischer-Tropsch reactor to at least one of a feed stream being fed into a synthesis gas formation reactor or the synthesis gas being fed into the Fischer-Tropsch reactor. The process further comprises obtaining a naphtha from the Fischer-Tropsch product and introducing the naphtha into a naphtha reformer. The naphtha is then reformed producing hydrogen by-product and C6-C10 product. At least a portion of the hydrogen by-product is then fed into the feed stream fed into the synthesis gas formation reactor, converting at least a portion of the CO2 in the feed stream into additional CO, by a reverse water gas shift reaction. Finally, the additional CO is converted into hydrocarbons in the Fischer-Tropsch reactor.
In accordance with another aspect of the invention, a process for reducing CO2 emissions from a Fischer-Tropsch GTL facility includes introducing a synthesis gas comprising CO, H2 and CO2 into a Fischer-Tropsch reactor. Next, a Fischer-Tropsch process is performed on the synthesis gas to produce a Fischer-Tropsch product. The Fischer-Tropsch product is then separated into unreacted CO, H2 and CO2, a C1-C5 product having a hydrogen to carbon ratio of about 2.0, a naphtha and a C10+ product having a hydrogen to carbon ratio of about 2.0. The unreacted CO, H2 and CO2 is then recirculated into at least one of a feed stream being fed into a synthesis gas formation reactor that produces the synthesis gas or the synthesis gas being fed into the Fischer-Tropsch reactor. The naphtha is then reformed to generate hydrogen by-product and C6-C10 product with a hydrogen to carbon ratio of less than about 2.0. The hydrogen by-product is then mixed with the feed stream so that at least a portion of any CO2 emitted from the Fischer-Tropsch reactor and recirculated into the feed stream, or any CO2 otherwise present in the feed stream is converted into additional CO by a reverse water gas shift reaction. Finally, the additional CO is converted into hydrocarbons in the Fischer-Tropsch reactor.
According to yet another aspect of the invention, a process for reducing CO2 emissions from a Fischer-Tropsch GTL process includes introducing a synthesis gas comprising CO, H2 and CO2, into a Fischer-Tropsch reactor. Next, a Fischer-Tropsch process is conducted on the synthesis gas to obtain a Fischer-Tropsch product. The Fischer-Tropsch product is then introduced into a first separator wherein the product is separated into unreacted CO, H2 and CO2, and hydrocarbon products. At least a portion of the unreacted CO, H2 and CO2 is then directed back into at least one of a feed stream being fed into a synthesis gas formation reactor, that produces the synthesis gas or the synthesis gas being fed into the Fischer-Tropsch reactor. At least a portion of the hydrocarbon products, separated from the Fischer-Tropsch product, are then directed into a second separator. In the second separator, the hydrocarbon products are separated into C1-C5 product having a hydrogen to carbon ratio of at least about 2.0 and a C10+ product having a hydrogen to carbon ratio of about 2.0. In addition, a naphtha is separated from the hydrocarbon products in the second separator. At least a portion of the naphtha is fed into a naphtha reformer. Hydrogen by-product is generated by reforming the naphtha in the naphtha reformer to produce C6-C10 product having a hydrogen to carbon ratio of less than about 2.0. At least a portion of the hydrogen by-product, generated during naphtha reforming, is mixed with the feed stream so that at least a portion of the CO2 emitted by the Fischer-Tropsch process, or otherwise present in the feedstream, is converted into additional CO by a reverse water gas shift reacton fueled by the hydrogen by-product. Finally, the additional CO is converted into hydrocarbons in the Fischer-Tropsch reactor.